iMac 2011 27" Weird glitch/distortion after GPU upgrade

[janek49]

Hi everybody,

I've just upgraded this iMac 2011 from the original AMD card to a nVidia GTX 880M (also tried 770M and same result) and a very weird graphical glitch happens now.

The graphics seem distorted, it reminds me of an old CRT screen thats out of sync. The distortion is not random, for example the second picture generates the effect seen in the first. You can barely make out the details of the actual display content. You can for example see the taskbar at the bottom, the explorer window in the center, etc.

I've done some diagnosis and came to the following conclusion:

What works:
- macOS High Sierra with patched VBIOS (The EFI/first seconds of boot are also distorted, then in the middle it "fixes" itself)
- Windows (any version) with stock VBIOS and proper drivers

What does not work:
- Any later macOS, even with OCLP patches (no matter what VBIOS i use)
- EFI Boot Screen (Apple default/OCLP no difference)

All the above produce this garbled mess. Stock VBIOS just shows black screen on EFI/Any macOS.

I've tried different motherboards, same result. The cards are both 100% fine, I've ran benchmarks under Windows, all works.

Can anybody hint me in the right direction, maybe this is a known issue I am just unable to properly find here.

Many thanks in advance!

 

[janek49]

UPDATE: For anyone interested, I've got it to work. Here's a short summary in case anyone runs into the same issue:

The problem was my laziness of not connecting the internal LCD

(Tbf, I repair and sell a lot of iMacs, so I have this diagonostic setup ready without the metal case, and connecting the LCD outside is a major pain)

I guess the way the patched VBIOS work, the external display relies on the internal displays sync. After connecting the internal LCD, with the patch, the EFI screen immediately worked, as well as booting to mac OS Monetery.
(Ignore the black bar in the middle, it's just a faulty LCD, nothing with the GPU)

I then could install the Root Patches (the normal ones were not enough I suppose.. Which is why I could not get any clear picture beforehand) which enabled the GPU to also work with just the external display again (with the same behavior of the EFI/first seconds of boot distorted, then fixing itself).

However now under Windows it looks weird again, less distorted, but something still ain't right, I guess it will need some driver patches...